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Pnmtojpeg User Manual(0) Pnmtojpeg User Manual(0)
NAME
pnmtojpeg - convert PNM image to a JFIF ('JPEG') image
SYNOPSIS
pnmtojpeg [-exif=filespec] [-quality=n] [{-grayscale|-greyscale}]
[-density=nxn[dpi,dpcm]] [-optimize|-optimise] [-rgb] [-progressive]
[-comment=text] [-dct={int|fast|float}] [-arithmetic] [-restart=n]
[-smooth=n] [-maxmemory=n] [-verbose] [-baseline] [-qtables=filespec]
[-qslots=n[,...]] [-sample=HxV[,...]] [-scans=filespec]
[-tracelevel=N]
filename
Minimum unique abbreviation of option is acceptable. You may use dou-
ble hyphens instead of single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1)
pnmtojpeg converts the named PBM, PGM, or PPM image file, or the stan-
dard input if no file is named, to a JFIF file on the standard output.
pnmtojpeg uses the Independent JPEG Group's JPEG library to create the
output file. See http://www.ijg.org <http://www.ijg.org> for infor-
mation on the library.
'JFIF' is the correct name for the image format commonly known as
'JPEG.' Strictly speaking, JPEG is a method of compression. The image
format using JPEG compression that is by far the most common is JFIF.
There is also a subformat of TIFF that uses JPEG compression.
EXIF is an image format that is a subformat of JFIF (to wit, a JFIF
file that contains an EXIF header as an APP1 marker). pnmtojpeg cre-
ates an EXIF image when you specify the -exif option.
OPTIONS
The basic options are:
-exif=filespec
This option specifies that the output image is to be EXIF (a
subformat of JFIF), i.e. it will have an EXIF header as a JFIF
APP1 marker. The contents of that marker are the contents of
the specified file. The special value - means to read the EXIF
header contents from standard input. It is invalid to specify
standard input for both the EXIF header and the input image.
The EXIF file starts with a two byte field which is the length
of the file, including the length field, in pure binary, most
significant byte first. The special value of zero for the
length field means there is to be no EXIF header, i.e. the same
as no -exif option. This is useful for when you convert a file
from JFIF to PNM using jpegtopnm, then transform it, then con-
vert it back to JFIF with pnmtojpeg, and you don't know whether
or not it includes an EXIF header. jpegtopnm creates an EXIF
file containing nothing but two bytes of zero when the input
JFIF file has no EXIF header. Thus, you can transfer any EXIF
header from the input JFIF to the output JFIF without worrying
about whether an EXIF header actually exists.
The contents of the EXIF file after the length field are the
exact byte for byte contents of the APP1 marker, not counting
the length field, that constitutes the EXIF header.
-quality=n
Scale quantization tables to adjust image quality. n is 0
(worst) to 100 (best); default is 75. Below about 25 can pro-
duce images some interpreters won't be able to interpret. See
below for more info.
-grayscale
-greyscale
-rgb These options determine the color space used in the JFIF output.
-grayscale (or -greyscale) means to create a gray scale JFIF,
converting from color PPM input if necessary. -rgb means to
create an RGB JFIF, and the program fails if the input is not
PPM.
If you specify neither, The output file is in YCbCr format if
the input is PPM, and grayscale format if the input is PBM or
PGM.
YCbCr format (a color is represented by an intensity value and
two chrominance values) usually compresses much better than RGB
(a color is represented by one red, one green, and one blue
value). RGB is rare. But you may be able to convert between
JFIF and PPM faster with RGB, since it's the same color space
PPM uses.
The testimg.ppm file that comes with Netpbm is 2.3 times larger
with the -rgb option than with the YCbCr default, and in one
experiment pnmtojpeg took 16% more CPU time to convert it. The
extra CPU time probably indicates that processing of all the
extra compressed data consumed all the CPU time saved by not
having to convert the RGB inputs to YCbCr.
Grayscale format takes up a lot less space and takes less time
to create and process than the color formats, even if the image
contains nothing but black, white, and gray.
The -rgb option was added in Netpbm 10.11 in October 2002.
-density=density
This option determines the density (aka resolution) information
recorded in the JFIF output image. It does not affect the
raster in any way; it just tells whoever reads the JFIF how to
interpret the raster.
The density value takes the form xxy followed by an optional
unit specifier of dpi or dpcm. Examples: 1x1, 3x2, 300x300dpi,
100x200dpcm. The first number is the horizontal density; the
2nd number is the vertical density. Each may be any integer
from 1 to 65535. The unit specifier is dpi for dots per inch or
dpcm for dots per centimeter. If you don't specify the units,
the density information goes into the JFIF explicitly stating
"density unspecified" (also interpreted as "unknown"). This may
seem pointless, but note that even without specifying the units,
the density numbers tell the aspect ratio of the pixels. E.g.
1x1 tells you the pixels are square. 3x2 tells you the pixels
are vertical rectangles.
Note that if you specify different horizontal and vertical den-
sities, the resulting JFIF image is not a true representation of
the input PNM image, because pnmtojpeg converts the raster
pixel-for-pixel and the pixels of a PNM image are defined to be
square. Thus, if you start with a square PNM image and specify
-density=3x2, the resulting JFIF image is a horizontally
squashed version of the original. However, it is common to use
an input image which is a slight variation on PNM rather than
true PNM such that the pixels are not square. In that case, the
appropriate -density option yields a faithful reproduction of
the input pseudo-PNM image.
The default is 1x1 in unspecified units.
Before Netpbm 10.15 (April 2003), this option did not exist and
the pnmtojpeg always created a JFIF with a density of 1x1 in
unspecified units.
-optimize
Perform optimization of entropy encoding parameters. Without
this, pnmtojpeg uses default encoding parameters. -optimize
usually makes the JFIF file a little smaller, but pnmtojpeg runs
somewhat slower and needs much more memory. Image quality and
speed of decompression are unaffected by -optimize.
-progressive
Create a progressive JPEG file (see below).
-comment=text
Include a comment marker in the JFIF output, with comment text
text.
Without this option, there are no comment markers in the output.
The -quality option lets you trade off compressed file size against
quality of the reconstructed image: the higher the quality setting, the
larger the JFIF file, and the closer the output image will be to the
original input. Normally you want to use the lowest quality setting
(smallest file) that decompresses into something visually indistin-
guishable from the original image. For this purpose the quality set-
ting should be between 50 and 95 for reasonable results; the default of
75 is often about right. If you see defects at -quality=75, then go up
5 or 10 counts at a time until you are happy with the output image.
(The optimal setting will vary from one image to another.)
-quality=100 generates a quantization table of all 1's, minimizing loss
in the quantization step (but there is still information loss in sub-
sampling, as well as roundoff error). This setting is mainly of inter-
est for experimental purposes. Quality values above about 95 are not
recommended for normal use; the compressed file size goes up dramati-
cally for hardly any gain in output image quality.
In the other direction, quality values below 50 will produce very small
files of low image quality. Settings around 5 to 10 might be useful in
preparing an index of a large image library, for example. Try -qual-
ity=2 (or so) for some amusing Cubist effects. (Note: quality values
below about 25 generate 2-byte quantization tables, which are consid-
ered optional in the JFIF standard. pnmtojpeg emits a warning message
when you give such a quality value, because some other JFIF programs
may be unable to decode the resulting file. Use -baseline if you need
to ensure compatibility at low quality values.)
The -progressive option creates a 'progressive JPEG' file. In this
type of JFIF file, the data is stored in multiple scans of increasing
quality. If the file is being transmitted over a slow communications
link, the decoder can use the first scan to display a low-quality image
very quickly, and can then improve the display with each subsequent
scan. The final image is exactly equivalent to a standard JFIF file of
the same quality setting, and the total file size is about the same --
often a little smaller.
Caution: progressive JPEG is not yet widely implemented, so many
decoders will be unable to view a progressive JPEG file at all.
If you're trying to control the quality/file size tradeoff, you might
consider the JPEG2000 format instead. See pamtojpeg2k(1)
Options for advanced users:
-dct=int
Use integer DCT method (default).
-dct=fast
Use fast integer DCT (less accurate).
-dct=float
Use floating-point DCT method. The float method is very
slightly more accurate than the int method, but is much slower
unless your machine has very fast floating-point hardware. Also
note that results of the floating-point method may vary slightly
across machines, while the integer methods should give the same
results everywhere. The fast integer method is much less accu-
rate than the other two.
-arithmetic
Use arithmetic coding. Default is Huffman encoding. Arithmetic
coding tends to get you a smaller result.
You may need patent licenses to use this option. According to
the JPEG FAQ <http://www.faqs.org/faqs/jpeg-faq> , This method
is covered by patents owned by IBM, AT&T, and Mitsubishi.
The author of the FAQ recommends against using arithmetic coding
(and therefore this option) because the space savings is not
great enough to justify the legal hassles.
Most JPEG libraries, including any distributed by the Indepen-
dent JPEG Group since about 1998 are not capable of arithmetic
encoding. pnmtojpeg uses a JPEG library (either bound to it
when the pnmtojpeg executable was built or accessed on your sys-
tem at run time) to do the JPEG encoding. If pnmtojpeg termi-
nates with the message, 'Sorry, there are legal restrictions on
arithmetic coding' or 'Sorry, arithmetic coding not supported,'
this is the problem.
-restart=n
Emit a JPEG restart marker every n MCU rows, or every n MCU
blocks if you append B to the number. -restart 0 (the default)
means no restart markers.
-smooth=n
Smooth the input image to eliminate dithering noise. n, ranging
from 1 to 100, indicates the strength of smoothing. 0 (the
default) means no smoothing.
-maxmemory=n
Set a limit for amount of memory to use in processing large
images. Value is in thousands of bytes, or millions of bytes if
you append M to the number. For example, -max=4m selects
4,000,000 bytes. If pnmtojpeg needs more space, it will use
temporary files.
-verbose
Print to the Standard Error file messages about the conversion
process. This can be helpful in debugging problems.
The -restart option tells pnmtojpeg to insert extra markers that allow
a JPEG decoder to resynchronize after a transmission error. Without
restart markers, any damage to a compressed file will usually ruin the
image from the point of the error to the end of the image; with restart
markers, the damage is usually confined to the portion of the image up
to the next restart marker. Of course, the restart markers occupy
extra space. We recommend -restart=1 for images that will be transmit-
ted across unreliable networks such as Usenet.
The -smooth option filters the input to eliminate fine-scale noise.
This is often useful when converting dithered images to JFIF: a moder-
ate smoothing factor of 10 to 50 gets rid of dithering patterns in the
input file, resulting in a smaller JFIF file and a better-looking
image. Too large a smoothing factor will visibly blur the image, how-
ever.
Options for wizards:
-baseline
Force baseline-compatible quantization tables to be generated.
This clamps quantization values to 8 bits even at low quality
settings. (This switch is poorly named, since it does not
ensure that the output is actually baseline JPEG. For example,
you can use -baseline and -progressive together.)
-qtables=filespec
Use the quantization tables given in the specified text file.
-qslots=n[,...]
Select which quantization table to use for each color component.
-sample=HxV[,...]
Set JPEG sampling factors for each color component.
-scans=filespec
Use the scan script given in the specified text file. See below
for information on scan scripts.
-tracelevel=N
This sets the level of debug tracing the program outputs as it
runs. 0 means none, and is the default. This level primarily
controls tracing of the JPEG library, and you can get some
pretty interesting information about the compression process.
The 'wizard' options are intended for experimentation with JPEG. If
you don't know what you are doing, don't use them. These switches are
documented further in the file wizard.doc that comes with the Indepen-
dent JPEG Group's JPEG library.
EXAMPLES
This example compresses the PPM file foo.ppm with a quality factor of
60 and saves the output as foo.jpg:
pnmtojpeg -quality=60 foo.ppm > foo.jpg
Here's a more typical example. It converts from BMP to JFIF:
cat foo.bmp | bmptoppm | pnmtojpeg > foo.jpg
JPEG Loss
When you compress with JPEG, you lose information -- i.e. the resulting
image has somewhat lower quality than the original. This is a charac-
teristic of JPEG itself, not any particular program. So if you do the
usual Netpbm thing and convert from JFIF to PNM, manipulate, then con-
vert back to JFIF, you will lose quality. The more you do it, the more
you lose. Drawings (charts, cartoons, line drawings, and such with few
colors and sharp edges) suffer the most.
To avoid this, you can use a compressed image format other than JPEG.
PNG and JPEG2000 are good choices, and Netpbm contains converters for
those.
If you need to use JFIF on a drawing, you should experiment with pnmto-
jpeg's -quality and -smooth options to get a satisfactory conversion.
-smooth 10 or so is often helpful.
Because of the loss, you should do all the manipulation you have to do
on the image in some other format and convert to JFIF as the last step.
And if you can keep a copy in the original format, so much the better.
The -optimize option to pnmtojpeg is worth using when you are making a
'final' version for posting or archiving. It's also a win when you are
using low quality settings to make very small JFIF files; the percent-
age improvement is often a lot more than it is on larger files. (At
present, -optimize mode is automatically in effect when you generate a
progressive JPEG file).
You can do flipping and rotating transformations losslessly with the
program jpegtran, which is packaged with the Independent Jpeg Group's
JPEG library. jpegtran exercises its intimate knowledge of the way
JPEG works to do the transformation without ever actually decompressing
the image.
Another program, cjpeg, is similar. cjpeg is
maintained by the Independent JPEG Group and packaged with the JPEG
library which pnmtojpeg uses for all its JPEG work. Because of that,
you may expect it to exploit more current JPEG features. Also, since
you have to have the library to run pnmtojpeg, but not vice versa,
cjpeg may be more commonly available.
On the other hand, cjpeg does not use the NetPBM libraries to process
its input, as all the NetPBM tools such as pnmtojpeg do. This means it
is less likely to be consistent with all the other programs that deal
with the NetPBM formats. Also, the command syntax of pnmtojpeg is con-
sistent with that of the other Netpbm tools, unlike cjpeg.
SCAN SCRIPTS
Use the -scan option to specify a scan script. Or use the -progressive
option to specify a particular built-in scan script.
Just what a scan script is, and the basic format of the scan script
file, is covered in the wizard.doc file that comes with the Independent
JPEG Group's JPEG library. Scan scripts are same for pnmtojpeg as the
are for cjpeg.
This section contains additional information that isn't, but probably
should be, in that document.
First, there are many restrictions on what is a valid scan script. The
JPEG library, and thus pnmtojpeg, checks thoroughly for any lack of
compliance with these restrictions, but does little to tell you how the
script fails to comply. The messages are very general and sometimes
untrue.
To start with, the entries for the DC coefficient must come before any
entries for the AC coefficients. The DC coefficient is Coefficient 0;
all the other coefficients are AC coefficients. So in an entry for the
DC coefficient, the two numbers after the colon must be 0 and 0. In an
entry for AC coefficients, the first number after the colon must not be
0.
In a DC entry, the color components must be in increasing order. E.g.
'0,2,1' before the colon is wrong. So is '0,0,0'.
In an entry for an AC coefficient, you must specify only one color com-
ponent. I.e. there can be only one number before the colon.
In the first entry for a particular coefficient for a particular color
component, the 'Ah' value must be zero, but the Al value can be any
valid bit number. In subsequent entries, Ah must be the Al value from
the previous entry (for that coefficient for that color component), and
the Al value must be one less than the Ah value.
The script must ultimately specify at least some of the DC coefficient
for every color component. Otherwise, you get the error message
'Script does not transmit all the data.' You need not specify all of
the bits of the DC coefficient, or any of the AC coefficients.
There is a standard option in building the JPEG library to omit scan
script capability. If for some reason your library was built with this
option, you get the message 'Requested feature was omitted at compile
time.'
ENVIRONMENT
JPEGMEM
If this environment variable is set, its value is the default
memory limit. The value is specified as described for the
-maxmemory option. An explicit -maxmemory option overrides any
JPEGMEM.
SEE ALSO
jpegtopnm(1) , pnm(5) , cjpeg man page, djpeg man page, jpegtran man
page, rdjpgcom man page, wrjpgcom man page
Wallace, Gregory K. 'The JPEG Still Picture Compression Standard',
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
AUTHOR
pnmtojpeg and this manual were derived in large part from cjpeg, by the
Independent JPEG Group. The program is otherwise by Bryan Henderson on
March 07, 2000.
netpbm documentation 23 April 2007 Pnmtojpeg User Manual(0)
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